She said: “William slipped in to the party after the other guests had arrived.”

Prince William’s own 21st celebrations were held at Windsor Castle that summer.

He chose an “Out of Africa” theme for his party, and guests attended in costume.

Ms Moody also revealed how the Queen joined in, “dressed as the Queen of Swaziland in a glittering white gown and African headdress.”

She said: “The whole venue was decorated to look like the African jungle, with huge model elephants the corners tribal masks on the walls and a lifelike replica giraffe’s head above the bar, while a band called Shakiramba were flown over from Botswana, and William took to the stage to play drums.”